[InstCombine] Negator: 'or' with no common bits set is just 'add'

In `InstCombiner::visitAdd()`, we have
```
  // A+B --> A|B iff A and B have no bits set in common.
  if (haveNoCommonBitsSet(LHS, RHS, DL, &AC, &I, &DT))
    return BinaryOperator::CreateOr(LHS, RHS);
```
so we should handle such `or`'s here, too.